1. Field of the Invention
The present invention relates to an optical communications network comprising a main station and a plurality of processing nodes and employing wavelength division multiplexing. Preferably, the present invention concerns optical network employed in a mobile radio communications network.
2. Description of the Related Art
As is known, at the main hierarchical levels of a conventional cellular mobile radio communications network there are the following: a switching center for processing the calls of mobile radio communications traffic and for managing the interface with the public telephone network, radio base stations connected and controlled by the switching center, and mobile stations (or terminals) in communication with the radio base stations.
In general, the technological choice forming the basis of cellular networks consists in associating the various radio base stations with a respective area or cell within the territory. Moreover, in principle, each cell has associated with it a group of radio channels which may also be reused in another cell, situated at given distance from the first cell and if necessary also adjacent thereto, depending on the type of mobile radio communications system.
According to conventional architecture of cellular networks, the radio base stations, which are located in sites spread over the territory covered by the mobile radio communications system, have the function of performing suitable processing of signals received from the mobile terminals (uplink) or to be transmitted thereto (downlink). In particular, according to conventional architectures, the radio base station houses those apparatus which have the main functions of receiving/transmitting voice or data signals and which perform coding or decoding thereof in accordance with a particular mobile radio communications system used, for example, the UMTS system (Universal Mobile Telecommunication System).
A particular type of mobile radio communications network uses optical fiber links between suitable nodes or units of the network. This type of communication is termed Radio over Fiber (RoF) and, generally, it can be divided into two different typologies of communications systems: RoF systems based upon analog transmission over fiber and RoF systems based upon digital transmission over fiber.
In connection with the first above mentioned typology, document WO-A-2004/019524 describes a wireless communications network including base station hotel and a remote cell site wherein multiple uplink and downlink signals are transmitted using a single optical fiber. According to this document, an analog multiplexing RF (Radio Frequency) transport technique is employed. At the base station hotel a number of downlink RF signals are frequency translated to corresponding downlink IF (Intermediate Frequency) signals having N distinct intermediate frequencies. The resulting frequency shifted signals are then combined and the resulting IF signal is converted to an optical signal centered at wavelength λd. This optical signal is transmitted over a single optical fiber from the base station to the remote site where it is converted into an IF signal and split into its N components centered at distinct IF frequencies. These IF components are then frequency shifted to a common carrier frequency, thereby recovering the original downlink RF signals generated by the base station. These N downlink signals are then transmitted from the antennas at the remote site to cellular subscribers in the cell's service area.
Moreover, U.S. Pat. No. 6,353,600 describes a CDMA (Code Division Multiplexed Access) cellular communications network in which centralized traffic channel resources are distributed to remote cells by use of WDM (Wavelength Division Multiplexing) on optical fibers. In accordance with this documents, the remote cells include optical add-and-drop multiplexers which selectively drop respective downlink optical signals from the optical fibers. In the above patent, it is stated that there is a predetermined one-to-one correspondence between the selected downlink optical wavelengths and directional antenna in a remote cell.
In connection with the digital RoF systems, U.S. Pat. No. 5,642,405 describes a microcellular communications network including a plurality of base station units and corresponding remote units provided with antennas. The remote units are connected through fibers to respective base station units using wave division multiplexing. A base station unit comprises transmitter banks having an output connected to an analog-to-digital converter which digitizes and frames radiofrequency signals and applies an optical output to a wavelength division multiplexer/demultiplexer, which is in turn connected to the fiber. This wavelength division multiplexer/demultiplexer allows to apply the optical information received from the remote units to an optical filter which filters out the signal received from the remote units as distinct optical wavelength.
Document WO-A-2004/0474472 describes a main-remote radio base station system including a plurality of N remote radio units connected in a series configuration by a single optical fiber along which the information between the main unit and the remote units propagates. According to this document, a data distribution approach over the single fiber is used to avoid the expenses of the wavelength division multiplexing technologies including laser, filters OADM, as well as the logistical overhead needed to keep track of different wavelength dependent devices. The main unit disclosed by WO-A-2004/0474472 combines N words of data, one word corresponding to each of the N remote units, into a frame and transmits the frame on the fiber. From the received frame, each remote unit removes its corresponding data words, includes an uplink word in the removed word place, and passes the frame to the next remote unit.